1. Technical Field
The present invention relates to a method of producing a polarization-maintaining optical fiber with a core having an elliptic cross-sectional shape.
2. Background Art
The use of a polarization-maintaining optical fiber as a transmission line for coherent light communication has been contemplated.
The polarization-maintaining optical fiber propagates light of HE.sub.11 mode which is polarized in two major axial directions orthogonal to each other. In order to maintain the polarized state under disturbances such as bending, pressure, etc., the difference in propagation constant between two HE.sub.11 modes is set to be large, thereby suppressing mode coupling. There are two typical methods of providing the propagation constant difference: one method &lt;1&gt; is by making the core noncircular in cross section, and the other &lt;2&gt; is by forming a stressed portion in the vicinity of the core.
FIG. 16 shows various types of polarization-maintaining optical fiber according to the prior art. In the figure, (1) is the panda type, (2) is the bow tie type, (3) shows the elliptic jacket type, (4) is the side tunnel type, (5) and (6) show the flat cladding type, and (7) is the elliptic core type. At present, the types in which the core is true circular in cross section and stressed ((1) to (6) in FIG. 16) are most commonly practiced, and the types of (5) and (6) in FIG. 16 in which the cladding is flat have been contemplated for improvement of coupling properties for optical integrated circuits.
The elliptic core type polarization-maintaining optical fiber ((7) in FIG. 16), based on the above-mentioned method &lt;1&gt;, is advantageous in that a large preform can be obtained and a long optical fiber with good polartization characteristic can be obtained. This type of optical fiber, however, is not so studied recently. The reason is that, with the elliptic core type, it is difficult to realize the noncircular cross-sectional shape on an industrial basis while maintaining the low-loss characteristics.
The polartization-maintaining optical fibers of (5) and (6) in FIGURE ]6 are disadvantageous in that, due to the neccessity for not only the step of providing the flat cross-sectional profile but the step of stressing the core, the production process is complicated, and elaborate equipment is required for the production. Therefore, it is impossible to obtain inexpensively these types of polarization-maintaining optical fiber.